The present invention relates to the determination of size and/or landing position of droplets of fluid ejected from a fluid ejection device. Some examples of various types of fluid ejection devices include inkjet printheads, medical devices, fuel injectors and other applications where droplets are to be forcefully ejected from a device such as a piezo-electric, thermal or any other fluid droplet ejector under controlled conditions. For convenience embodiments will be described with reference to inkjet printers which typically use thermal or piezo-electric means to forcefully eject ink droplets through microscopic orifices onto media on which printing is to take place.
Inkjet printers are of various types including those on which one or more inkjet printheads, also known as pens, are mounted on a scanning carriage and others in which the printheads may be mounted in stationary position on a frame for so-called page-wide-array (PWA) printing. Scanning or reciprocating inkjet printers ordinarily have a printhead servicing station located at some point on the path of travel of a printhead carriage, typically to one side or the other of the print area, so that the scanning carriage and associated printheads thereon can be moved to the service station for spitting, priming, wiping, capping or otherwise servicing the printhead orifices. The servicing station may include printhead wipers, a source of printhead servicing fluid and printhead caps, some or all of which may be mounted in a stationary position or on a sled or other moveable support to bring the printheads to be serviced and the service station components into and out of operating proximity to each other for servicing. Inkjet printers with stationary printheads or pens which also may require periodic servicing may employ such a sled or moveable support to bring the service station to the stationary printheads when servicing of the printhead orifices is required.
In the art of inkjet printing, the accuracy of placement of the individual ink droplets which form the printed image is typically tested by printing a test pattern of droplets onto the print media and visually selecting the best-matched pattern(s). In automatic systems, printed test sheets are evaluated by optically measuring the position of selected points of the printed pattern for comparison with stored data representative of the desired position of selected points of the pattern to generate printhead error correction control signals to adjust the firing of ink from the various orifices of the pen. Known methods for doing so involve detecting the landing positions or sizes of the individual droplets after they have been printed onto a sheet of test media and have the disadvantage that the process interrupts printing, wastes media and takes several seconds, because the steps of printing and measuring are done in series. In high speed printers, every second is critical to system throughput and, in addition, the accuracy of photo-sensors used to detect position and size of droplets is dependent not only upon the type of photosensor used but also upon the media type. Furthermore, sensor reliability may degrade as ink and/or ink aerosol accumulates in the sensor window.
Other earlier techniques include real time optical measurement of the in-flight trajectory of the inkjet droplets from at least two different directions orthogonal to the direction of droplet flight, in order to calculate true trajectory to develop printhead error correction control signals. This type of apparatus is very expensive and is typically used only in a controlled or test environment.
Low cost ink drop sensing techniques are disclosed in U.S. Pat. No. 6,086,190 issued Jul. 11, 2000 to Schantz, et al, owned by the assignee of the present invention, by electrically charging ink drops in flight which impact on an electrostatic sensor to determine whether printheads are firing ink and the volume and or velocity of ink drops fired in bursts. The landing positions of the fired drops are not determinable by these techniques.
In one embodiment, the present invention provides a method of determining performance of a fluid ejection device, comprising:
arranging said device relative to a sensor array of droplet detection transducers for projecting droplets from said device onto said array;
projecting at least one droplet onto said array;
detecting the outputs of each of said transducers to provide signals representative of fluid from said droplet on each of said transducers; and
processing said signals to determine the size and/or position of said droplet.
Another embodiment of the invention provides a method of adjusting performance of a fluid ejection device in which droplets are projected from said device through space onto a target, comprising:
arranging said device and target comprised of a droplet detecting array of fluid responsive transducers in relative position for projecting droplets from said device onto said array;
projecting at least one droplet onto said array;
detecting outputs of at least some of said transducers to provide signals representative of fluid from said droplet on said some of said transducers in said array; and
using said signals to adjust the ejection of a subsequent droplet to impinge onto desired location on said target.
Various embodiments of a droplet detection device having a droplet reception surface comprising an array of spaced fluid responsive transducers are also disclosed, each transducer being capable of providing a signal indicative of presence of fluid from a droplet thereon.